1. Field of the Invention
This invention relates to a clutch/brake unit for use in a riding lawnmower or similar vehicle, and more particularly, to a unit having a pole piece having a relatively high magnetic reluctance separating a pair of pole pieces having relatively low magnetic reluctance to enable more efficient release of the brake and engagement of the clutch.
2. Disclosure of Related Art
Clutch/brake units are commonly adapted for use in riding lawnmowers, garden tractors or similar vehicles to selectively transmit a drive torque from a motor to an implement drive (e.g., a mower blade drive) and to apply a positive braking torque to the drive when drive torque is not being transmitted.
A conventional clutch/brake unit may include an input member, or hub, connected to a longitudinally extending input shaft for rotation therewith, a rotor disc mounted on the input member, an armature axially spaced from the rotor disc, and an output member such as a pulley connected to the armature. The armature and output member may be connected by a plurality of springs that bias the armature in a first axial direction away from the rotor disc and into a braked position against a braking member. The unit may further include an electromagnetic clutch disposed on a side of the rotor disc opposite the armature. The clutch may include an electrical coil disposed within a stationary field shell. Energization of the coil establishes a magnetic circuit between the field shell, rotor disc, and armature and draws the armature in a second axial direction towards the rotor disc and into a clutch engaged position--concurrently releasing the armature from engagement with the braking member(s). Deenergization of the coil severs the magnetic circuit and the springs once again draw the armature in the first axial direction into a braked position against the braking member.
As noted by applicant in applicant's prior U.S. Pat. No. 5,119,918 (hereinafter, "'918 patent")--the entire disclosure of which is incorporated herein by reference--the above-described clutch/brake units have significant drawbacks. In these conventional units, the entire braking force is provided by the springs that connect the armature and the output member. As a result, these springs must be relatively strong. A relatively high electromagnetic force must then be generated by the electromagnetic clutch in order to release the brake and cause engagement of the clutch. In addition, the force provided by the springs increases as the armature is drawn away from the braked position and towards the clutch-engaged position against the rotor disc. Because a high electromagnetic force is required, larger and/or more expensive electrical components must be used both within the clutch/brake unit itself and within any vehicle or machine that incorporates the unit.
In the '918 patent, applicant disclosed a clutch/brake unit that is capable of overcoming the above-mentioned deficiencies. In particular, applicant disclosed a clutch/brake unit that incorporates a plurality of permanent magnets to effectuate braking. The plurality of magnets are disposed between a radially extending flange of the field shell and a corresponding plurality of pole pieces having relatively low magnetic reluctance. The magnets create a magnetic circuit between the magnets, the pole pieces, the field shell, and the armature that draws the armature in a first axial direction into a braked position against the pole pieces. Because the armature is drawn into a braked position by magnetic attraction, the springs that connect the armature and the output member need only be strong enough to transmit torque from the armature to the output member. Moreover, the braking force provided by the magnetic circuit decreases as the armature is drawn in a second axial direction towards the rotor disc and into the clutch-engaged position (as opposed to the increasing force in the conventional spring-set brake). Because less force is required to release the brake and bring the armature into engagement with the rotor disc, smaller and/or less expensive electrical components can be used in the clutch/brake unit and in the vehicle or machine incorporating the unit.
Although the clutch/brake unit described in the '918 patent represents a significant improvement over the prior art, additional improvements are desirable. One desired improvement would be to further reduce the electromagnetic force required to release the brake and engage the clutch. In the clutch/brake unit disclosed in the '918 patent, the magnets and pole pieces span an angular distance of less than one half the circumference of the field shell. Therefore, when the coil is energized, a portion of the armature furthest from the magnets quickly snaps into engagement with the rotor disc. As the magnetic attraction between the armature and rotor disc increases, the portion of the armature adjacent the magnets also snaps into engagement with the rotor disc. The force required to bring this latter portion of the armature into engagement with the rotor disc, however, is still greater than desired.
Another desirable improvement would be to enable the application of a protective coating, such as chrome carbide, to one or more of the pole pieces in order to increase the durability and lifespan of the pole pieces. Chrome carbide has a high magnetic reluctance. In conventional, clutch/brake units, therefore, only a relatively small amount can be applied to the pole pieces without a significant reduction in the magnetic attraction between the armature and the pole pieces, and consequently, the braking torque.
There is thus a need for a brake that will minimize or eliminate one or more of the above-mentioned deficiencies.